The present invention relates to mobile radio telecommunication systems. More particularly, and not by way of limitation, the present invention is directed to a system and method for improving broadcast, paging, and synchronization performance for a cellular communication system by exploiting the benefits of macro-diversity.
Cellular systems send broadcast signals that are received by mobile stations to obtain important information that is used for proper system operation. Some of this information is system-specific, such as the system ID, the operator name, the services supported, and so on. Some of the information is cell-specific, such as the maximum power to be used by mobile stations to access the cell, and so on. In general, all broadcast information is sent independently on each cell in the entire cellular network belonging to that operator. In GSM, this information is sent on the Broadcast Control Channel (BCCH) or the Packet Broadcast Control Channel (PBCCH). Similar common channels exist for CDMA and WCDMA systems. System-specific as well as cell-specific information is sent on every cell, and bandwidth resources are separately allocated in each cell for the purpose of broadcast.
Conventional cellular systems are designed mainly for unicast services, wherein point-to-point communication is the primary goal, and is typically handled within a single cell where the mobile station is present. All other functionalities are built to support the primary objective, and are thus designed within the purview of a single cell. For example, in GSM, CDMA2000, and WCDMA, in order to support voice and data calls, means are provided in each cell for the mobile to synchronize to a suitable cell it has selected, and to obtain the broadcast information sent by the base station in that cell. Following such synchronization and reading of broadcast-information, the mobile station can access the system and set up communication links.
In GSM, each cell has a Frequency Correction Channel (FCH), which enables coarse frequency and time synchronization to the cell and provides a pointer to the Synchronization Channel (SCH), which enables finer synchronization to the cell. The SCH allows identification of the cell and a pointer to the BCCH. The BCCH contains all the broadcast information relevant to the system and the cell, and directs further point-to-point communication in the cell. Each of these logical channels is present in each individual cell. The FCH is the same signal in all cells, but a terminal can only synchronize to the FCH of one particular cell. Thus, the benefit of having the same FCH in every cell is essentially helpful only in lowering the search space of the terminal.
In CDMA2000, a common pilot channel is used for initial synchronization. The same common pilot channel is used in all cells, but a variable offset (n*64 chips) distinguishes cells. A terminal attempting to synchronize to the common pilot channel gets connected to a cell with a particular offset. Even a 64-chip offset, leave alone a multiple of 64, is too large for typical path search windows, and it is unlikely that it can be exploited for path combining from different base stations. A synchronization channel that provides further information is closely associated with the common pilot channel and provides a pointer to the broadcast control channel that provides additional information to the mobile station for further communication.
In WCDMA, synchronization is achieved by means of a Primary Synchronization code that is common to all base stations. Since base stations are typically asynchronous, it is most likely that a terminal synchronizes to one particular cell. Further, a secondary synchronization code provides information on frame boundaries, and indicates a group of scrambling codes. By searching the group of scrambling codes, the terminal identifies the cell and is in a position to receive the system broadcast information.
Macro-diversity is defined as the reception of similar information from a variety of radio links that are separated by a significant spatial distance between transmission sources. The receiver can improve the quality of the received signal by suitably combining the signals from these links. The term “similar information” is to be understood to refer to the ability to embed the same information, encoded optionally in differing ways, as all or part of two or more radio transmissions. A significant spatial distance, as applied to the qualifier, “macro,” is meant to denote cases where the transmitting radio sources are separated by distances including large fractions of the cell size, as well as capable of encompassing several base station sites. Encoding information in this regard pertains to operations such as scrambling, interleaving, or channel encoding and combinations thereof.
A common way of performing macro-diversity is to transmit the exact same information from multiple transmitters at substantially the same time. The receiver receives a sum of signals that have passed through different radio links, and uses appropriate demodulation methods to obtain a performance benefit. One benefit of this way of performing macro-diversity is that the receiver is not substantially different from a receiver that is designed to receive a signal from only one transmitter.
In the GSM, CDMA2000, and WCDMA systems discussed earlier, it is clear that even if a synchronization signal that is essentially similar is present across multiple cells, the systems are not configured to permit the use of macro-diversity to enhance system synchronization. The uniformity of the signals is present just as a means to simplify initial synchronization.
More recently, there has been significant interest in offering broadcast services over the cellular network, wherein the same signal is broadcast to many users across multiple cells. This has lead to services such as Multimedia Broadcast/Multicast Service (MBMS) for WCDMA and broadcast TV services, such as Digital Video Broadcast-Handheld (DVB-H), to handheld terminals. Since these services are broadcasting information that is possibly common to all users, these services use methods such as macro-diversity to help improve the performance of information delivery to users in the system. To date, these broadcast techniques have been used for broadcasting services. Indeed, the design imperatives that have been used for such services may be extended in a novel direction for the purpose of broadcasting system information as well, even in a system whose primary purpose is point-to-point communication.
Solutions in use for broadcasting system information in conventional cellular systems cannot exploit any of the advantages provided by macro-diversity because the information is different from cell to cell. The current art needs an improved system and method for delivering all the relevant broadcast information to mobile stations operating in a cellular communication system, while at the same time allowing the use of point-to-point services to deliver data to particular users. The present invention provides such a system and method.